Seasonal variations in uptake and in situ regeneration of nitrogen in mangrove waters
Limnol. Oceanogr., 47(1), 2002, 241-254 | DOI: 10.4319/lo.2002.47.1.0241
ABSTRACT: Seasonal changes of uptake of nitrogenous nutrients (NH4+, NO3-, NO2-, and urea) and regeneration (NH4+ and NO2- production) in the plankton fraction of a mangrove ecosystem on the west coast of India were investigated. Nitrate was the major fraction of assimilable N (72%), followed by NH4+ (16%), NO2- (6%), and urea (6%). Changes of nutrient concentrations followed clear seasonal cycles and were mainly regulated by in situ biological processes. The plankton took up NO3- and NH4+ in more or less equal proportions (39 and 44% respectively), followed by urea (11%) and NO2- (6%). Seasonal patterns of uptake were distinct, with a dominance of NO3- and NO uptake in the postmonsoon, followed by a dominance of NH4+ and urea uptake in the premonsoon. The high and prolonged use of NO3- at the beginning of the productive season was due to a strong allochthonous supply of NO3-, dominance of microplankton, and low NH4+ regeneration rates. Heterotrophs may take up all four nutrients and could account for half of the annual total N uptake. Ammonium and NO2- regeneration rates were among the highest known from nearshore waters and showed clear seasonal patterns. Production and use of NH4+ were closely coupled. Nitrite production rates were related to NH4+ production rates in a rectangular-hyperbolic fit. Nitrogen balance analyses showed that proximity to mangrove vegetation enhanced the flux rates, noninclusion of nitrification may lead to an overestimation of new production by 30%, and regeneration in the plankton fraction provided about 40% more N than was assimilated.